Reliance on the International Law and international cooperation in general is only way how to survive in international asmosphere which tends to be either hegemonic or anarchic. Any of these conditions is not suitable for less powerfull states.
Beba Bajalski reason is ineffective sanction mechanism towards them. If you take out sanctions from structure or reality of legal norm it disputably remains legal norms in true sense. It that sense it is disputable if "powerfull states" (at least permanent members of the UN SC) are indeed binded by the International Law, resp. are over it. On the other hand argument in favor of the IL is that even powerfull states are using legal rethoric and they themself consider as subjects to the IL.
, there is an economic benefit for poorer countries to contribute to UN peacekeeping. Let me give you an example, troop-contributing countries pay for their military personnel's salary - using their own national formula. The UN, however, does reimburse each country a standard rate of $1,428 per soldier per month. In 2012, Bangladesh spent around $4,500 per year or $375 per month on each of their troops. Thus, contributing troops benefits their military budgets. In comparison, Canadians spend around $130,000 per year (2012 figures). Thus the math does not make sense. When Canada contributes their troops to UN Peacekeeping they are doing for other reasons - a commitment to humanitarianism, etc... For more information on the political economy of peacekeeping, see Sandler (2017) in the Journal of Conflict Resolution or Gaibulloev et al (2015) in the Journal of Peace Research.
The benefits are not only economic. Many countries' training regimes are very limiting. Participation in UN peacekeeping gives these troops exposures to best practices in military affairs, important connections with other officer corps, and so forth. They also get valuable military experience, which cannot be ignored.
Thus, benefits are not always economic in nature. But having talked to many officials in UN peacekeeping operations over the years, I can tell you that for smaller countries it is an important part of their calculus. Indeed, small or poorer countries with big militaries can subsidize their expenditures by participating in UN operations - this is, of course, kind of ironic as the UN also calls on its members to spend less money on military expenditures.
To echo on the above arguments, I think poor countries look at the cost-benefit analysis. In that, the more troops we contribute the more benefits they yield to our country. It goes back to the issue of dependency...
Small states do not have the power to deter wars waged by large countries, so they contribute to peace projects as an attempt to avoid wars and conflicts, i.e. used or under control by the force of international law because they do not possess material or real power, which is an ideal and moral approach.
I think small states pay attention to peacekeeping missions, because they know how fragile is the wellbeing of their own country and how in a matter of second everything can change. Since most of these countries go first from peacebuilding then to peacekeeping, they recognize how painful would it be if they don't invest in peacekeeping process, dialogue and reconciliation. And let alone the fact that in order to be a member of important international organisations, you have to bear some responsibility and maintain law order in accordance with international law standards.